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Effect of Chemical Warm Mix Additive on the Properties and Mechanical Performance of Recycled Asphalt Mixtures
Effect of Chemical Warm Mix Additive on the Properties and Mechanical Performance of Recycled...
Barraj, Firas;Khatib, Jamal;Castro, Alberte;Elkordi, Adel
buildings Article Effect of Chemical Warm Mix Additive on the Properties and Mechanical Performance of Recycled Asphalt Mixtures 1 1 , 2 , 3 1 , 4 Firas Barraj , Jamal Khatib * , Alberte Castro and Adel Elkordi Faculty of Engineering, Beirut Arab University, Beirut 11-5020, Lebanon; firstname.lastname@example.org (F.B.); email@example.com (A.E.) Faculty of Science and Engineering, University of Wolverhampton, Wolverhampton WV1 1LY, UK Department of Agroforestry Engineering, Transportation Infrastructures and Engineering, Higher Polytechnic School of Engineering, University of Santiago de Compostela, Campus University s/n, 27002 Lugo, Spain; firstname.lastname@example.org Department of Civil and Environmental Engineering, Faculty of Engineering, Alexandria University, Alexandria 21511, Egypt * Correspondence: email@example.com Abstract: Newer technologies such as warm mix asphalt (WMA) and reclaimed asphalt pavement (RAP) have gained international approval and have been considered as appropriate solutions that support the sustainability goals of the highway sector. However, both technologies present some shortcomings. The lower mixing and compaction temperatures of WMA reduce the binder aging and the bond between the aggregates and the coating binder, thus resulting in less rutting resistance and higher moisture susceptibility. On the other hand, RAP mixes tend to be stiffer and more brittle than conventional hot mix asphalt (HMA) due to the effect of aged binder. This tends to increase the crack propagation distresses. In an attempt to overcome their individual shortcomings, this study investigated the new concept of a combined WMA-RAP technology. The chemical WMA additive Rediset LQ1102CE was utilized with mixtures incorporating low (15%), medium (25%), and Citation: Barraj, F.; Khatib, J.; high (45%) RAP contents. Dynamic modulus (DM) and ﬂow number (FN) tests were conducted Castro, A.; Elkordi, A. Effect of to investigate the effect of Rediset on the behavior of RAP mixtures. The dynamic modulus |E*| Chemical Warm Mix Additive on the mastercurves were developed using the sigmoidal model and Franken model was used to ﬁt the Properties and Mechanical accumulated permanent deformation curve. The results of this study showed that Rediset addition Performance of Recycled Asphalt Mixtures. Buildings 2022, 12, 874. improved the cracking resistance of RAP mixtures. However, the rutting resistance was reduced but https://doi.org/10.3390/ kept within the acceptable range except for mixtures containing low RAP content. buildings12070874 Keywords: reclaimed asphalt pavement; warm mix; recycled; sustainability; dynamic modulus; Academic Editor: Pengfei Liu energy consumption Received: 20 May 2022 Accepted: 14 June 2022 Published: 21 June 2022 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Newer technologies such as warm mix asphalt (WMA) and reclaimed asphalt pave- published maps and institutional afﬁl- ment (RAP) have gained international approval and have been considered as appropriate iations. solutions that support the sustainability goals of the highway sector. Warm mix technology lowers the production and compaction temperatures which contributes to a reduction in the amount of greenhouse gases emitted and energy consumption [1,2]. Reclaimed asphalt technology promotes the recycling of old pavement materials to be used as a replacement Copyright: © 2022 by the authors. of virgin binder and aggregates for the production of new asphalt mixtures. Moreover, RAP Licensee MDPI, Basel, Switzerland. technology leads to the preservation of non-renewable resources, reduction of production This article is an open access article costs, and other economic and environmental beneﬁts . However, both technologies distributed under the terms and present some shortcomings. The lower mixing and compaction temperatures of WMA conditions of the Creative Commons reduce the binder aging and the bond between the aggregates and the coating binder, thus Attribution (CC BY) license (https:// resulting in less rutting resistance and higher moisture susceptibility [4–6]. On the other creativecommons.org/licenses/by/ hand, RAP mixes tend to be stiffer and more brittle than conventional HMA mixtures due 4.0/). Buildings 2022, 12, 874. https://doi.org/10.3390/buildings12070874 https://www.mdpi.com/journal/buildings Buildings 2022, 12, 874 2 of 21 to the effect of aged binder. Typically, in recycled pavement mixtures, with the admixing of aged asphalt, the blended asphalt is harder, has higher elasticity, and lower viscos- ity. The high-temperature performance improves, but the low-temperature performance worsens [7,8]. This tends to increase the crack propagation distresses [9,10]. In order to overcome these shortcomings, WMA-RAP technology has been investigated [11,12]. The aged binder of RAP might efﬁciently compensate for the soft warm mix binder, so that the fatigue life of RAP mixes gets extended and the rutting performance of WMA mixes is maintained. Ultimately, the most desirable feature of WMA-RAP mixes is the reduction in production and compaction temperatures while incorporating increased proportion of RAP leading to overall saving in energy and economical cost without compromising on the properties of the bituminous mix . A number of studies have been conducted in recent years to understand the perfor- mance of WMA-RAP mixtures. Oliveira et al.  indicated that the rutting resistance of HMA and RAP mixes was improved with the addition of a surfactant-based additive, how- ever, the effect was mostly recognized in RAP mixtures. Gungat et al.  used a multiple stress creep and recovery (MSCR) test to investigate the effect of RH-WMA (combination of polyethylene and wax) on asphalt mixtures incorporating 30, 40, and 50% RAP, and noticed an improvement in the rutting resistance with the increase in RAP content. Zhao et al.  conducted asphalt pavement analyzer, Hamburg wheel tracking, and ﬂow number tests on asphalt samples mixed with a foaming WMA additive, and incorporating RAP contents of 15% and 30%. They found that increased RAP content contributes to a decrease in the rut depth regardless of the source of reclaimed materials. Similar ﬁndings were reported by Xie et al. . Table 1 presents the effect of different WMA additives on the rutting performance of mixtures containing RAP. Table 1. Effect of WMA additives on the rutting resistance of RAP mixtures. Additive Rutting Resistance of Reference Additive Type RAP % Mix Design Characteristics Dosage % WMA-RAP Mixture Organic additive Dense graded mixtures Sasobit addition showed (Sasobit) prepared acording to Behbahani et al., better rutting resistance than chemical 25, 50, 70 Marshall method and 2017  Zycotherm Increase anti-stripping agent 3 designed for Va of 4% (Zycotherm) Dense graded mixtures Chemical additive compacted using the gyratory Evotherm 3G showed better compactor following rutting resistance than Lu et al., 2016  25, 50, 70 (Evotherm 3G) 0.5 Australian standards at Sylvaroad TM RP 1000 Sylvaroad TM RP optimum binder content for 1000 * 2 Va of 4% Surfactant additive Dense graded mixtures SI WMA-RAP showed better 5.3 (SI) prepared according to rutting resistance than 20, 40 Guo et al., 2016  Chemical additive Marshall method for a Va of Evotherm DAT WMA-RAP (Evotherm DAT) 4.5% Dense graded mixtures Organic (Sasobit) Evotherm and Sasobit 1.5 compacted using Superpave WMA-RAP presented the gyratory compactor 0.5 same rutting resistance, while Kim et al., 2017  Chemical (Evotherm) according to AASHTO T Advera additive cause a 342-11 for Va ranging from [0.25] relatively poor performance Foaming (Advera) 4.7% to 7.4% Dense graded mixtures Rutting depth decreased by Sharkawy et al., Organic wax prepared according to 1.5; 3 15 11.91% if Sasobit additive 2016  (Sasobit) Marshall method for Va content increased by 1.5% between 3% and 5% Semi-dense asphalt mixtures Foaming The rutting depth of Zeolite prepared following Marshall Valdes-Vidal et al., with 0.6% content was 5 mm [0.3]; [0.6] 20 method according to Chiliean 2018  shallower than that of Zeolite standard for Va ranging Technique (Zeolite) with 0.3% between 4.5% and 5.3%. Note:  indicates that dosage % is by weight of the mix while others are by weight of the binder; * product made from Crude Tall Oil and Crude Sulphate Turpentine. Buildings 2022, 12, 874 3 of 21 In addition to the rutting potential of WMA-RAP mixtures, the fatigue under repetitive loading and low temperature cracking need to be assessed. In that context, Das et al.  conducted dynamic modulus (DM), indirect tensile (IDT), and bending beam rheometer (BBR) tests to measure the stiffness, creep compliance, and tensile strength of WMA-RAP asphalt samples and showed that water foaming WMA and asphaltene B additives had no effect on the fatigue and thermal cracking performance of RAP mixes. Gabchi et al.  found that Zeolite-based additives (ZTWM, CHWM-B, and CHWM-S) enhance the creep compliance of RAP mixtures at 18 C which implies a better stress relaxation and a better thermal cracking resistance for WMA-RAP. Vaitkus et al.  reported a decline in the cracking performance of WMA-RAP mixtures when Sasobit dosage exceeds 2%. In another study by Singh et al. , the results of the semi-circular bending (SCB) testing indicated that the addition of Ft-wax additive reduced the fracture resistance of RAP mixes. Zhao et al.  found that the cracking performance of asphalt mixtures containing up to 30% RAP content could be improved by the WMA foaming technology; however, when 30% threshold is exceeded, the performance is compromised. Another concern regarding WMA-RAP mixtures is its moisture susceptibility, es- pecially for foamed techniques that work by injecting water to lower the virgin binder viscosity. Low mixing temperature feature of WMA, non-ductile RAP binder, and aggre- gate striping concerns of RAP technology are main factors that create the necessity to investigate the moisture susceptibility of WMA-RAP produced mixtures. For this reason, to assess WMA additive impact, Frigio et al.  conducted indirect tensile strength ITS, Cantabro, SCB, and repeated indirect tensile tests on mixes containing 15% of RAP and using different WMA additives and found that the chemical additive provides acceptable resistance to moisture while the organic wax and zeolite performed poorly. The chemi- cal additive performed best because of the inherent antistripping capabilities. Moreover, Guo et al.  indicated that the short-term aged WMA-RAP mixtures provided higher tensile ratio TSR moisture resistance than the corresponding RAP mixture. However, TSR values were drastically reduced after long-term aging. Further studies should be conducted to characterize the effect of the diversity of avail- able WMA additives on the properties and mechanical performance of asphalt mixtures produced through WMA-RAP technology in order to determine the best additive and with which RAP content. In this regard, the work of this paper addresses this goal and discusses the impact of a chemical WMA additive (Rediset LQ1102CE ) on the performance of as- phalt mixtures incorporating low (15%), medium (25%), and high (45%) RAP contents. The Rediset effect on reclaimed asphalt mixtures needs to be investigated. Rediset additive can be considered as an easy-to-use liquid that not only is a WMA additive, but it also provides an active adhesion that enhances the coating of aggregates and improves the moisture resistance of asphalt mixtures [30,31]. Therefore, the objectives of this paper are to: (a) Determine the effect of the chemical WMA additive Rediset on the dynamic modulus and phase angle properties of the asphalt mixture containing different RAP percentages, (b) evaluate the impact of Rediset addition on the permanent deformation and the fatigue behavior of RAP mixtures, and (c) investigate the correlations between the ﬂow number (FN) and dynamic modulus (DM) results used to characterize the asphalt mix rutting resistance potential. 2. Experimental 2.1. Materials Raw materials consisting of limestone aggregates, unmodiﬁed binder with a per- formance grade PG 64-22, and RAP aggregates from one single source were used in the fabrication of all the mixtures of the study. The average binder contents of ﬁne and coarse RAP fractions determined following the centrifugal extraction method of ASTM D2172M and the average particle size distribution of extracted aggregates from each RAP stockpile are illustrated in Table 2 and Figure 1. The evaluation of the recovered binder of recycled pavement materials showed RAP binder to be graded at PG 88-4. Increased performance Buildings 2022, 12, 874 4 of 21 grade for RAP binder properties is the result of asphalt oxidation, volatilization, and steric hardening, which are involved in the asphalt’s long-term aging process . Table 2. Average binder content of coarse and ﬁne RAP laboratory samples. Standard Sample ID 1 2 3 4 Average Deviation RAP + 4.75 mm 3.66 3.50 3.64 4.05 3.71 0.236 RAP 4.75 mm 5.01 5.40 4.98 5.17 5.14 0.192 Figure 1. Particle size distribution of RAP fractions before and after extraction of binder (BE and AE). The binder from RAP was assumed to be totally blended with the virgin binder as recommended by Superpave Technical Report . Therefore, the amount of RAP binder and the after extraction (AE) curves of RAP aggregates for both RAP fractions were considered in the mix design. Binder replacement ratios of 13.02%, 21.6%, and 38.37% were found for mixes with 15%, 25%, and 45% RAP contents, respectively. It should be noted that the mixing conditions were optimized as suggested by Abed et al.  to maximize the degree of blending DOB between virgin and RAP binders so that the full blending assumption is reasonable. In this context, the mixing time was extended to 6 min for all mixes and the mixing temperature was maintained at 135 C for warm asphaltic mixtures. For WMA-RAP mixtures prepared with reduced temperatures, the chemical WMA additive Rediset LQ1102CE , developed by Akzonobel (Amsterdam, The Netherlands) and which comes in the form of a liquid, was utilized. This type of additive reduces the friction between the aggregate and the asphalt binder that is coating it, improving the workability of AC mixtures during its production and compaction processes. 2.2. Preparation of Asphalt Mixtures In this study, six AC mixtures are assessed having aggregate gradation with a nominal maximum aggregate size (NMAS) of 12.5 mm designed to meet the control points criteria’s of Superpave (SP-2)  as illustrated in Figure 2. The labels “HR15”, “HR25”, “HR45”, “WR15”, “WR25”, and “WR45” are used in the upcoming sections to designate, respectively, HMA mix with 15% (low RAP content), 25% (medium RAP content), and 45% (high RAP content), 15% (low RAP content + Rediset), 25% (medium RAP content + Rediset), and 45% (high RAP content + Rediset). Buildings 2022, 12, 874 5 of 21 Figure 2. Gradation selected for asphalt mixtures. It is worth noting that in order to obtain grading curves for mixes containing RAP that are close to a control gradation, the proportions for coarse and ﬁne RAP materials in the mixture were adjusted according to the percentage of RAP incorporation as shown in Table 3. Table 3. Coarse and ﬁne RAP proportions in RAP and WMA-RAP mixes. % RAP in Mix 15% 25% 45% % Coarse fraction 11.82 19.71 35.47 % Fine fraction 3.18 5.29 9.53 Mix Design and Compaction The mix design for “HR15”, “HR25”, and “HR45” mixtures of the study was conducted to determine the optimum binder content OBC (% by weight of the mix) that provides a design air voids Va of 4%. Then, the WMA-RAP mixes were designed by validating the OBC that was determined for each of the corresponding HR mixes. In order to design a WMA-RAP mix with a speciﬁc volume of air voids, three different factors can be varied: compaction temperature, asphalt content, and/or WMA additive dosage. The same OBC of HR mix was used for the corresponding WMA-RAP mix to obtain similarity of volu- metric properties so that a comparable performance can be achieved between HR and its corresponding WR mix . Moreover, this study adopted the same temperature reduction of 25 C for both the mixing and compaction of the WMA-RAP mixes. The supplier of Rediset additive recommends a dosage between 0.3% and 0.7% by weight of the binder and a reduction range in the mixing and compaction temperatures between 20–40 C. The level of temperature reduction was targeted to satisfy the condition of ensuring a full coating of the aggregates and a 4.0% air voids for the same compaction effort used for HMA. For each WMA-RAP mix, different Rediset dosages were tried such that they fell within the range recommended by the supplier in order to determine the appropriate dosage for each level of RAP incorporation. A summary of the mix design parameters for all mixtures is shown in Table 4. Buildings 2022, 12, 874 6 of 21 Table 4. Summary of mix design results for HR and WR mixes. Superpave Mix HR15 HR25 HR45 WR15 WR25 WR45 Criteria OBC 4.6 4.62 4.68 4.6 4.62 4.68 Va % 4.0 4.0 4.0 4.1 3.94 4.02 4 VMA % 14.01 14.0 14.125 14.05 14.01 14.09 14% min VFA% 71.58 71.39 71.65 71.45 71.4 71.55 65–75 % Gmm at Nini 85.77 85.56 85.14 85.95 85.88 85.64 89% max % Gmm at Nmax 97.43 96.99 97.48 97.63 97.29 97.73 98% max Rediset dosage - - - 0.5 0.4 0.31 % Dust proportion (DP) 0.7 0.71 0.7 0.72 0.71 0.71 0.7–1.2 Mixing temperature C 160 160 160 135 135 135 Compaction temperature C 150 150 150 125 125 125 The results of the mix design showed that the volumetric properties for HR and WR mixes are comparable and acceptable according to Superpave mix design criteria. Moreover, it is seen that the optimum binder content to achieve air voids volume Va of 4% did not vary by more than 0.08% between the mixes with low and high RAP contents indicating efﬁcient blending between virgin and RAP binder occurred at the chosen mixing temperature. 2.3. Testing 2.3.1. Complex Modulus Test The complex modulus test (E*) is an unconﬁned test conducted to characterize the stiff- ness measured in term of the dynamic modulus |E*|, and viscoelastic behavior exhibited by the phase angle F of asphalt mixtures over a range of different temperatures and load- ing frequencies. The National Cooperative Highway Research Program (NCHRP) report 547 of the NCHRP Project 9–19 recommended |E*| as one of the most promising simple performance tests for rutting and fatigue cracking speciﬁcation because of its signiﬁcant correlation with these distresses . Higher |E*| at small, intermediate, and large frequen- cies correlated, respectively, to better rutting resistance, worst fatigue, and worst thermal cracking resistance and vice versa . In this test, asphalt concrete (AC) samples with height of 150 mm and diameter of 100 mm are subjected to repetitive sinusoidal dynamic compressive axial load (stress) as shown in Figure 3. The stress amplitude is controlled to produce a recoverable strain not exceeding a maximum amplitude of 70 microstrains to keep the material response in the linear viscoelastic range . For this study, three replicates of each AC mixture were tested at three temperatures, 5, 20, and 40 C, and six loading frequencies, 20, 10, 5, 1, 0.5, and 0.1 Hz, following the Aashtoo T 342-11 protocol. The averages of the |E*| and F of the three specimens at each temperature/frequency combination were calculated. Then, the dynamic modulus and phase angle mastercurves were constructed by applying the time temperature superposition (TTS) principle at a reference temperature of 20 C. The dynamic modulus |E*| mastercurves were developed using the sigmoidal model of Equation (1) as a function of the reduced frequency f which in turn is a function of real frequency f and temperature T as shown in Equations (2) and (3). Log E = d + (1) (j j) b+g log ( f ) 1 + e log f = log f + log (a ) (2) Log a = a T + a T + a (3) ( ) T 1 2 3 Buildings 2022, 12, 874 7 of 21 where d, a, b, and g are regression constants found by numerical minimization between measured and ﬁtted values of log|E*|. d, a, (b and g) represent, respectively, the min- imum value of |E*|, vertical span of the modulus function, and shape parameters, and a , a , and a represent regression coefﬁcients of the polynomial model used to ﬁt the shift 1 2 3 factor function log (a ) at any particular temperature. Figure 3. Complex modulus testing in the asphalt mixture performance tester (AMPT). Dynamic Modulus Mastercurves Figure 4 presents an example of construction of the mastercurve of the RAP mixture “HR15” by shifting measured |E*| values horizontally over the frequency axis. Figure 4. Dynamic modulus mastercurve of RAP mix “HR15” at 20 C in (a) semi-log scale, (b) log-log scale. Phase Angle Mastercurves The phase angle F is deﬁned as the delay between the applied peak stress and the peak strain response under cycling loading . F is needed to calculate the storage and the loss modulus components of the complex modulus. It also provides direct insight into the magnitude of viscous and/or elastic state of the AC mixture. The higher the phase angle measured, the lower is the elasticity of AC, and thus more viscous behavior governs. Buildings 2022, 12, 874 8 of 21 F = 0 for a purely elastic material and 90 for a purely viscous material. To construct the F mastercurve, a similar approach to the |E*| mastercurve can be adopted. Moreover, since AC is a thermorheologically simple TRS material , F values measured at a particular temperature can be shifted horizontally to the reference temperature using the same shift factors found while developing |E*| mastercurve. Figure 5 shows a typical F versus f graph for the RAP mixture “HR15” at a reference temperature of 20 C. Figure 5. Phase angle construction for the “HR15” mixture. It is recognized that the shifted data of F forms approximately a single smooth curve, indicating that the shift factors from |E*| can be successfully used to construct the F mastercurve. Very few models exist in the literature intended to ﬁt a F mastercurve. The most common are the Booji and Thoone  approximate model, and its modiﬁed version proposed by Yang and You  expressed in Equations (4) and (5), respectively. b+g log ( f ) F f = 90 ag (4) ( ) b+g log ( f ) 1 + e b+g log ( f ) F ( f ) = c 90 ag (5) b+g log ( f ) 1 + e where F ( f ) is the phase angle in degrees, f is the reduced frequency in Hz, a, b, g are the r r ﬁtting parameters found in the sigmoidal function of the dynamic modulus presented in Equation (1), and c is a regression parameter found by numerical minimization between measured and ﬁtted values of F. As shown in Figure 6, the F mastercurve ﬁtted using the aforementioned models lacked some accuracy in predicting the phase angle. Thus, another modiﬁcation was proposed to Yang and Lou model as expressed in Equation (6). b+g log ( f ) 3 2 F ( f r) = k [log( f )] + k [log( f )] + k [log( f )] k 90 ag (6) 1 r 2 r 3 r 4 b+g log ( f ) 1 + e Buildings 2022, 12, 874 9 of 21 where k , k , k , k are constant regression parameters. 1 2 3 4 Figure 6. Phase angle mastercurves ﬁtted by the three models for RAP mix “HR15”. As it can be seen, the proposed model accurately predicted the F mastercurve of RAP mix “HR15”. Furthermore, Figure 7 illustrates an example of the phase angle mas- tercurves ﬁtted using Equation (6) for HR and WR mixes. It is observed that good ﬁts are obtained, thus, the phase angle mastercurves of various mixes of the study predicted using Equation (6) were used to analyze the effect of Rediset on the phase angle of mixtures incorporating different RAP percentages. Figure 7. Phase angle mastercurves ﬁtted using Equation (9) for: (a) HR 25%, (b) WR 25%. 2.3.2. Flow Number (FN) Test The ﬂow number test is a simple performance test (dynamic creep and recovery test) introduced by the NCHRP 9–19 Project to assess the rutting potential of asphalt mixes . Buildings 2022, 12, 874 10 of 21 In this test, AC samples with height of 150 mm and diameter of 100 mm are subjected to haversine compressive cyclic loading, each cycle consisting of a load pulse of 0.1 s long followed by a 0.9 s rest period, to measure the accumulated axial permanent strains as a function of loading cycles. For this study, FN testing was conducted according to Aashtoo T378-17 using a peak amplitude of the repeated axial stress equal to 600 Kpa under a temperature of 53 C to simulate the pavement surface conditions during summer in regions with hot climate. The test was set to terminate at 10,000 loading cycles or accumulated 7% permanent strain, whichever came ﬁrst. Figure 8 shows a typical plot of the accumulation of the permanent strain under a given set of material, load repetitions and environmental conditions. Three zones of permanent deformation are identiﬁed: (1) Primary zone, predominantly attributed to volumetric change, in which high initial level of rutting occurs and the rate of plastic deformations decreases as the number of cycles increases; (2) secondary zone, attributed predominately to volumetric changes in addition to plastic deformations induced by shear stresses, in which a small level of rutting is encountered and the deformation rate is approximately constant with loading cycles; and (3) tertiary zone, predominantly associated with plastic (shear) deformations under no volume change conditions, in which high level of rutting is exhibited and the rate of deformation increases rapidly with loading cycles. Figure 8. Typical repeated load vs. permanent strain (deformation) behavior of pavement materials . Model Used to Identify the Flow Number The ﬂow number FN is deﬁned as the number of load cycles at which the asphalt mixture reaches the ﬂow point, i.e., the start of the tertiary zone. FN, expressed in cycles, corresponds to the point where the slope of the permanent strain curve reaches a minimum value. A stable and rational approach to determine FN was recommended by Researchers at the Arizona State University (Tempe, AZ, USA) , in which the permanent axial strain is initially ﬁtted using the Franken model expressed by Equation (7) below. B Dn " = An +C e 1) (7) where " is the permanent axial strain, n is the number of cycles, and A, B, C, and D are regression coefﬁcients found by numerical optimization (minimization of error) between measured and ﬁtted values of permanent strains per each cycle. Once ﬁtted, the ﬁrst and second derivatives of the Francken model can be easily calculated analytically as shown in Buildings 2022, 12, 874 11 of 21 Equations (8) and (9), respectively, and the ﬂow number is located as the cycle where the second derivative changes from negative to positive sign. (B 1) Dn = ABn +CDe (8) d " (B 2) 2 Dn = AB B 1)n +CD e (9) An example on the curve ﬁtting of the measured permanent axial strain using the Franken model is illustrated by Figure 9 for one replicate of “HR15” mixture and the ﬂow number FN is determined at 294 cycles. Figure 9. Flow number determination using Franklin model for ﬁtting the measured accumulated permanent strain (%) curve for one replicate of the “HR15” mixture. Although the ﬂow number has been widely accepted as an indicator of the rutting resistance , Zhang et al.  suggested that a FN Index parameter provides more balanced evaluation of the rutting performance of asphalt mixtures since it takes into consideration both the strains and the number of load repetitions endured till entering the tertiary ﬂow. FN index equation is expressed by Equation (10) below. For performance analysis, higher values of FN indicate better rutting resistance whereas higher FN index values indicate a worst rutting resistance of AC mixtures. FN Index = (10) FN 3. Results and Analysis 3.1. Mastercurves Parameters For all mixes of the study, the summary of the ﬁtting parameters associated with the functions of |E*|, log at, and F is presented in Table 5. R statistics (above 0.99) indicates an excellent curve ﬁtting of the dynamic modulus and phase angle mastercurves to measured data utilizing the sigmoidal model given in Equation (1) and the polynomial shift factor function expressed in Equation (2), as well as the modiﬁed phase angle expression of Equation (6). Buildings 2022, 12, 874 12 of 21 Table 5. Summary of mastercurve (MC) ﬁtting parameters and statistics for various AC mixtures. Log at Versus T Function |E*| MC Parameters F MC Parameters Mix Parameters Type 2 2 2 R a a a R k k k k R 1 2 3 1 2 3 4 HR 15 0.9162 3.4227 1.5560 0.4976 0.9994 0.0012 0.1871 3.2797 1.0000 0.0868 0.3042 0.7548 1.1035 0.9939 HR 25 0.3057 4.0590 1.7698 0.4616 0.9994 0.0013 0.1983 3.4372 1.0000 0.1041 0.4267 1.1931 1.1342 0.9965 HR 45 0.0245 4.3699 1.8576 0.4464 0.9997 0.0013 0.1946 3.3768 1.0000 0.1416 0.5874 0.8227 1.1285 0.9975 WR15 0.9676 3.3350 1.4544 0.5208 0.9996 0.0013 0.1936 3.3341 1.0000 0.0423 0.1701 0.7423 1.0473 0.9973 WR25 0.9649 3.3405 1.5196 0.527 0.9991 0.001 0.1813 3.2268 1.0000 0.0933 0.3806 0.5469 1.0610 0.9971 WR45 0.0449 4.3838 1.8692 0.4636 0.9991 0.0011 0.1885 3.327 1.0000 0.1217 0.8457 2.1842 1.1564 0.9909 3.2. Dynamic Modulus Results Figures 10 and 11 show the dynamic modulus mastercurves of diverse mixes of the study in log-log and semi-log scale, respectively. Figure 10. Dynamic modulus mastercurves in semi-log scale. Figure 11. Dynamic modulus mastercurves in log-log scale. Buildings 2022, 12, 874 13 of 21 A quick analysis of the plots indicated that the higher the RAP content, the higher is the |E*| of the mixture for both RAP and WMA-RAP technology. Moreover, the values of modulus of all HR mixes are higher than those corresponding to WR mixes. Except f of 10 Hz, from the largest to smallest, over the full analyzed frequency range, |E*| of the mixtures can be mostly ranked as follow: HR45, HR25, HR15, WR45, WR25, and ﬁnally WR15. Speciﬁcally, the results showed that the addition of Rediset additive reduces |E*| of the corresponding HR mix at both high and low reduced frequencies. Because of the strong correlation between |E*| and the rutting and fatigue distresses , this reduction in |E*| implies that the addition of Rediset might affect the performance of AC mixtures. Effect of Rediset on |E*| According to RAP Content To highlight the effect of Rediset on each RAP content separately, the reduction (%) in |E*| against the reduced frequency is presented in Figure 12. For analysis purposes, the frequency range of |E*| mastercurves is decomposed into three arbitrary zones: (1) 5 2 2 +2 lower range from 10 to 10 Hz, middle range from 10 to 10 Hz, and a higher +2 +5 range from 10 to 10 Hz. It is worth noting that the behavior of AC mixtures at the lower, middle, and higher frequency ranges correspond to its behavior at regions with high, normal (operational), and low temperature ranges. Figure 12. Effect of Rediset on |E*| of mixtures with RAP. For HMA mixtures incorporating low RAP content (HR15), higher reduction due to Rediset addition occurred in the lower frequency range compared to the middle and higher range. This reduction is signiﬁcant and increased from 22.98% at 10 Hz to a maximum 3 2 of 26.97% at 10 Hz then decreased to 24.27% at 10 Hz. In the middle frequency 2 +2 range, the reduction in |E*| decreased from 24.27% at 10 Hz to 9.32% at 10 Hz. In the higher frequency range, the reduction in |E*| is almost constant and not signiﬁcant +2 +5 ranging between 9.32% at 10 Hz and 7.5% at 10 Hz. For HMA mixtures incorporating medium RAP content (HR25), the reduction is slight at very low frequency of 10 Hz but it becomes signiﬁcant and at the highest in this range 4 2 3 speciﬁcally between 10 and 10 Hz while having a peak of 27.1% at 10 Hz. In the 2 +2 middle range, the reduction in |E*| decreased from 25.74% at 10 Hz to 8.5% at 10 Hz. In the higher frequency range, the reduction in |E*| is almost constant and not signiﬁcant +2 +5 ranging between 8.5% at 10 Hz and 9.45% at 10 Hz. In the case of HMA mixtures with high RAP content (HR45), the reduction is signiﬁcant and occurred also in the lower range of frequencies with a maximum |E*| reduction of 27.85% at 10 Hz. As f increases in the lower range, the reduction decreased reaching 16.7% at 10 Hz. In the middle frequency range, the reduction in |E*| decreased from 2 +2 16.7% at 10 Hz to 9.88% at 10 Hz. In the higher frequency range, the reduction in Buildings 2022, 12, 874 14 of 21 |E*| is almost constant and not signiﬁcant ranging between 9.88% at 10 Hz and 10.69% at 10 Hz. As a summary, the reduction in |E*| for HR15 and HR25 caused by Rediset addition is almost equal and higher than that of HR45 in the middle range of frequencies. In the higher f range, comparable reduction in |E*| is found for different RAP mixtures assessed. Essentially, it was observed that the effect of Rediset on RAP mixtures was at the highest and mostly signiﬁcant in the lower frequency range regardless of the RAP content incorporated. Knowing that the behavior of AC mixtures at low frequencies corresponds to its behavior at high temperature in which the rutting distress prevails, it is very critical to assess the impact of the signiﬁcant reduction in |E*| found on the rutting performance of WMA-RAP mixtures compared to standard RAP mixtures. In addition, the reduction in |E*| caused by Rediset additive in both the middle and higher range of frequencies indicates, respectively, an improvement in the fatigue and ther- mal cracking of WMA-RAP mixtures with respect to their corresponding HMA-RAP mixes. 3.3. Phase Angle Results Figure 13 shows the phase angle mastercurves of diverse mixes of the study in semi- log scale. In the case of phase angle, the effect of the addition of Rediset was not consistent over the range of analyzed frequencies for RAP mixes unlike the case of the dynamic modulus. WR15 mix has the highest F at smaller frequencies and at the lower band of middle frequency range indicating highest viscous behavior of this mix among others which correlates well with the lowest dynamic modulus associated to it. At the upper band of the middle range and at higher frequencies, comparable F can generally be observed. Figure 13. Phase angle mastercurves in semi-log scale. Effect of Rediset on F According to RAP Content In order to show the effect of Rediset on each RAP content separately, the difference (%) in F between HR and its corresponding WR mix was plotted against the reduced frequency as illustrated by Figure 14. For HMA mixtures incorporating low RAP content (HR15), Rediset addition increased 5 +3 +3 F values between f of 10 and 10 Hz and decreased F values for f above 10 Hz. r r Nevertheless, the only signiﬁcant effect is observed at very low frequency of 10 Hz. Moreover, a slight increase of about 2.87% in the peak value of F is observed for WR15 compared to HR 15, while for both HR15 and WR15 mixes, the peak occurred approximately at the same reduced frequency level indicating that the maximum viscous effect occurs in both mixtures at the same temperature or at the same loading time. Buildings 2022, 12, 874 15 of 21 Figure 14. Effect of Rediset on F of mixtures with RAP. For HMA mixtures incorporating medium RAP content (HR25), Rediset addition 3 2 increased F values for f under 10 Hz, slightly decreased F values between 10 and +1 +1 10 Hz, and increased F values again above 10 Hz. However, the effect of Rediset is 5 +4 signiﬁcant only at very low frequency of 10 Hz and very high frequencies above 10 Hz. An insigniﬁcant increase of 1.7% in the peak value of WR25 compared to HR25 is observed, while for this WR25 mix, the peak F is reached at comparatively lower temperature (higher reduced frequency) than HR25, indicating development of peak viscous effect at relatively lower temperature for the AC mix. Finally, for HMA mixtures incorporating high RAP content (HR45), Rediset addition 2 1 increased F values for f under 10 Hz, decreased F values for f between 10 and r r +3 +4 10 Hz, and is ineffective on F values for f above 10 Hz. An insigniﬁcant decrease of 2.38% in the peak value of WR45 compared to HR45 is observed, while for this WR45 mix, the peak F is reached at comparatively lower temperature (higher reduced frequency) than HR45, indicating development of peak viscous effect at relatively lower temperature for the AC mix. 3.4. Flow Number Results Table 6 shows the results of FN testing for different HR and WR mixes and Figure 15 presents a graphical plot to compare both FN and FN index data. It is worth noting that the variation in the ﬂow number FN test can be relatively high as the highest proportion of the coefﬁcient of variation in FN-values was 28.97% for the mix HR45. The variation is even higher for the FN index calculation with a maximum COV value of 47.09% for HR45 as well. Table 6. Summary of ﬂow number (FN) test results. # AC Mix Sample ID# FN Cycles # FN Index replicate #1 294.00 18,475.00 62.84 replicate #2 271.00 19,203.00 70.86 replicate #3 421.00 23,678.00 56.24 1 HR15 Mean 328.67 20,452.00 62.23 Std 80.79 2817.41 7.32 COV (%) 24.58 13.78 11.76 Buildings 2022, 12, 874 16 of 21 Table 6. Cont. # AC Mix Sample ID# FN Cycles # FN Index replicate #1 343.00 21,821.00 63.62 replicate #2 479.00 21,128.00 44.11 replicate #3 376.00 20,305.00 54.00 2 HR25 Mean 399.33 21,084.67 53.91 Std 70.94 758.93 9.76 COV (%) 17.76 3.60 18.10 replicate #1 925.00 22,219.00 24.02 replicate #2 569.00 29,816.00 52.40 replicate #3 1037.00 26,391.00 25.45 3 HR45 Mean 843.67 26,142.00 33.96 Std 244.37 3804.62 15.99 COV (%) 28.97 14.55 47.09 replicate #1 165.00 24,213.00 146.75 replicate #2 130.00 25,190.00 193.77 replicate #3 138.00 13,765.00 99.75 4 WR15 Mean 144.33 21,056.00 146.75 Std 18.34 6333.06 47.01 COV (%) 12.71 30.08 32.03 replicate #1 224.00 22,186.00 99.04 replicate #2 203.00 25,668.00 126.44 replicate #3 184.00 24,231.00 131.69 5 WR25 Mean 203.67 24,028.33 119.06 Std 20.01 1749.82 17.53 COV (%) 9.82 7.28 14.72 replicate #1 247.00 17,168.00 69.51 replicate #2 181.00 24,764.00 136.82 replicate #3 212.00 22,352.00 105.43 6 WR45 Mean 213.33 21,428.00 103.92 Std 33.02 3881.38 33.68 COV (%) 15.48 18.11 32.41 From the analysis of the results both parameters indicate the same information, that WR mixes are more susceptible to rutting than HR mixes and by descending order, the rutting resistance of HR45 is the highest, then HR25, HR15, WR45, WR25, and ﬁnally WR15. Furthermore, it was found that the addition of Rediset reduced the FN of HR45 by 74.7%, HR25 by 49%, and HR 15 by 56%, and increased the FN index of HR 45, HR25, and HR15 by 136%, 121%, and 206%. Essentially, the results indicated that better rutting performance is associated to increased RAP content for both HMA-RAP and WMA-RAP mixtures. This can be explained by the fact that the RAP contains aged binder having higher PG grade that stiffen the mixture and is indeed contributing to an increase in the rutting resistance. Ultimately, the results of Figure 15 indicates that despite the reduction in FN induced by the Rediset effect, both RAP and WMA-RAP mixes except WR15 provided acceptable level of rutting performance with a FN above 190 cycles . Buildings 2022, 12, 874 17 of 21 Figure 15. Graphical comparisons of FN results. 3.5. Comparison between |E*| and FN Test Results of Asphalt Mixtures |E*| values at higher temperatures are generally used to evaluate the rutting per- formance since asphalt mixtures are susceptible to this distress at higher temperatures. In line with that, |E*| values at higher temperatures 37.8 C and 54 C with different frequency levels of 25, 10, 5, 1, 0.5, and 0.1 Hz were acquired from the dynamic modulus mastercurve of each mixture. Tables 7 and 8 show the results of |E*| at the investigated temperature/frequency condition versus the FN results for the mixtures of the study. Table 7. E*| at 37.8 C vs. FN results. Mix jEj Rank jEj Rank jEj Rank jEj Rank jEj Rank jEj Rank FN Rank 37.8 C,25 Hz 37.8 C,10 Hz 37.8 C,5 Hz 37.8 C,1 Hz 37.8 C,0.5 Hz 37.8 C,0.1 Hz HR15 3608.02 3 2700.77 3 2128.92 3 1159.58 3 875.73 2 426.36 3 3 HR25 3640.78 2 2730.56 2 2152.96 2 1163.96 2 871.54 3 445.03 2 2 HR45 4164.06 1 3160.14 1 2513.53 1 1382.48 1 1040.28 1 509.88 1 1 WR15 2844.59 6 2071.90 6 1599.90 6 834.54 6 621.03 6 309.32 6 6 WR25 3172.39 5 2325.44 5 1802.19 5 943.12 5 701.04 5 346.21 5 5 WR45 3383.45 4 2514.49 4 1965.24 4 1033.22 4 761.62 4 355.99 4 4 Table 8. E*| at 54 C vs. FN results. Mix jEj Rank jEj Rank jEj Rank jEj Rank jEj Rank jEj Rank FN Rank 54, 0.1 Hz 54 C,25 Hz 54 C,10 Hz 54 C,5 Hz 54 C,1 Hz 54 C,0.5 Hz HR15 1299.79 3 890.44 3 691.78 2 349.56 2 260.83 2 135.59 1 3 HR25 1330.47 2 922.09 2 659.17 3 316.78 3 229.15 3 108.38 3 2 HR45 1604.68 1 112.56 1 837.95 1 402.75 1 289.40 1 132.36 2 1 WR15 896.56 6 603.91 6 445.51 6 253.15 4 189.15 4 100.25 4 6 WR25 100.56 5 673.89 5 487.53 5 220.91 5 165.10 5 88.102 5 5 WR45 1013.10 4 684.201 4 507.43 4 220.44 6 155.18 6 68.832 6 4 It can be concluded from Table 7 that at a temperature of 37.8 C, DM and FN provide mostly similar ranking of the rutting performance of asphalt mixtures. However, as illustrated in Table 8, at a temperature of 54 C only at higher frequencies of 25 and 10 Hz, the ranking of rutting performance of DM and FN is comparable. At a temperature of 54 C and a frequency 5 Hz, DM ranks the rutting resistance of HR15 higher than HR 25 compared Buildings 2022, 12, 874 18 of 21 to FN. An additional difference is observed between DM and FN at a temperature of 54 C for smaller frequencies, especially at 0.1 Hz, where DM ranks, respectively, HR15 and WR45 as the highest and lowest resistant mixes to rutting whereas FN ranks HR 45 and WR15 as the highest and lowest resistant mixes to rutting. Graphical Correlations for the Laboratory Results The results of the correlations in Figure 16a reveal that at a temperature of 37.8 C at which both the dynamic modulus (DM) and ﬂow number (FN) present comparable ranking of the rutting potential of asphalt mixtures, strong correlations (R > 0.84) existed between |E*| and (FN) results, the best correlation being found at a frequency of 0.5 Hz and 0.1 Hz. At a temperature of 54 C, it is observed in Figure 16b that there are no strong correlations between the ﬂow number (FN) and |E*| values (R < 0.75) for frequencies of 1 Hz and below, however, strong correlations were found between the FN and |E*| values for higher frequencies of 5, 10, and 25 Hz (R > 0.86). Based on the ﬁndings of this section, and since the NCHRP 465 reported a strong correlation between FN results and rutting measured in ﬁeld , |E*| at 37.8 C and especially for 0.1 Hz (highest R = 0.857) or at 54 C and for a frequency of 10 Hz (highest R = 0.889) might be a proper DM laboratory test condition for estimating the rutting-resistance potential of RAP and WMA-RAP mixes in the ﬁeld. Figure 16. Correlations between FN and |E*| at (a) 37.8 C, (b) 54 C. Buildings 2022, 12, 874 19 of 21 4. Conclusions and Recommendations In order to evaluate the effect of Rediset additive on the performance of RAP mixtures, two commonly used laboratory performance tests, DM and FN, were conducted on six asphalt mixes incorporating different RAP contents. The main ﬁndings of this study are listed as follows: The addition of Rediset WMA additive reduces |E*| of the corresponding RAP mix at both high and low reduced frequencies. Nevertheless, the reduction is mostly signiﬁcant in the lower frequency range. The higher is the RAP content the higher is the |E*| of the mixture for both RAP and WMA-RAP technology, and along the frequency axis |E*| of RAP mixes with low, medium, and high RAP content are mostly higher than WMA-RAP mixes. The Rediset additive improved the fatigue and thermal cracking of WMA-RAP mix- tures with respect to their corresponding RAP mixes. On the other hand, the rutting resistance was reduced due to Rediset. Yet, it was acceptable for all mixes except the mix incorporating low RAP content of 15%. In the case of phase angle, the effect of the addition of Rediset was not consistent over the range of analyzed frequencies for RAP mixes. For HMA mixtures incorporating low RAP content (HR15), the only signiﬁcant effect of Rediset addition (increase in F) is observed at very low frequency of 10 Hz, above which comparable values of phase angle between RAP and WMA-RAP mix are obtained. For HMA mixtures incorporating medium RAP content (HR25), a signiﬁcant effect of Rediset addition (increase in F) is observed at very low frequency of 10 Hz and higher frequencies above 10 Hz, while comparable values of phase angles for RAP and WMA-RAP mix are obtained in the range between these frequencies. Regarding HMA mixtures incorporating high RAP content (HR45), Rediset addi- tion increased F values mostly for f under 10 Hz while for other frequencies, comparable phase angle values for RAP and WMA-RAP mixes can be expected. |E*| at 37.8 C and especially for 0.1 Hz or at 54 C and for a frequency of 10 Hz might be a proper DM test condition for estimating the rutting-resistance potential of RAP and WMA-RAP mixes in the ﬁeld. The ﬁndings revealed by this study can be elaborated by further investigation in areas that may include: 1. The effect of WMA additive on asphaltic mixtures incorporating RAP content exceed- ing 50%, 75%, and even 100% recyclable mixtures. 2. Assessing the impact of using vegetable, organic, or petroleum rejuvenator combined with the WMA additive on the properties and performance of HMA-RAP mixture. 3. Examination of the variation in the particle size distribution of the extracted recycled aggregates for mixtures produced using different RAP percentages. Author Contributions: Conceptualization, F.B.; methodology, F.B.; validation, J.K. and A.C.; formal analysis, F.B. and J.K.; investigation, F.B.; resources, F.B.; data curation, F.B. and A.C.; writing— original draft preparation, F.B.; writing—review and editing, J.K. and A.C.; visualization, F.B.; supervision, J.K. and A.E.; project administration, A.E. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: No applicable. Data Availability Statement: Data sharing not applicable. Acknowledgments: The authors of this manuscript appreciate the contribution of Hussein Kassem who provided us with useful information that helped in the completion of this study. Conﬂicts of Interest: The authors declare no conﬂict of interest. Buildings 2022, 12, 874 20 of 21 References 1. Prowell, B.D.; Hurley, G.C.; Frank, B. 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Effect of Chemical Warm Mix Additive on the Properties and Mechanical Performance of Recycled Asphalt Mixtures
, Volume 12 (7) –
Jun 21, 2022
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